Electrodes of this general nature have already been described in Belgian Patent Specification No. 867 876. The metal shank of electrodes described therein contains the cooling system or means and is covered by an externally disposed compound having a high temperature stability. Apparently such coating is continuous and hooks in the metal shank are provided to improve adhesion.
Similar electrodes are also claimed within the framework of British Patent Specification No. 1 223 162 in which the entire metal shank is covered with a protective ceramic coating. According to this proposal, efforts are made to ensure that the ceramic coating is as thin as possible and penetrates into the metal shank itself to provide a substantial degree of insulation for the tubes extending therein. These tubes simultaneously function for cooling ducting purposes and provide an electrical connection to the consumable electrode portion of graphite.
European Patent Application No. 79302809.3 describes an electrode in which the mechanical contact of the metal shank, laterally disposed externally, is supported so as to be insulated with respect to the internally disposed metallic cooling system. The bottom part of the metallic cooling shank is again provided with a ceramic coating, secured by hooks and extending approximately to the height of a screw nipple connection.
German Auslegeschrift No. 27 39 483 also describes an electrode of the above-mentioned kind in which liquid cooling is ensured employing annular ducts directly guided upon an outer wall. The liquid return preferably directly adjoins external surfaces of the metal shank so that the external wall of the metal shank also represents the internal wall of the return duct. The electrode includes two components or parts, the entire internal or inner part being removable from the external part of the top portion. To this end it is necessary to release the screw bolts of a ring flange and to lift out the internal structure after shutting down the supply of liquid and emptying the cooling system. In the event of damage to the region of the top portion this electrode does not however permit the adoption of rapid and relative simple means for repair. Moreover, mechanical damage of the top portion or a short circuit may lead to introduction of water into the arc furnace and consequent explosions due to the externally disposed annular duct and return duct.
Electrodes for arc furnaces are exposed to severe stresses. This is explained by elevated operating temperatures to which the electrode is exposed, for example in the production of electrode steel, for which such electrodes are most frequently employed. Losses due to side oxidation are also caused by the electric arc which extends into the melt away from the bottom electrode tip only in an ideal case. Finally, there is the risk of arc travelling or lateral striking of the arc which can also take place above the consumable part in the event of defects and thereby cause short circuits. Furthermore, the electrodes are subject to different temperatures in the feed and return of the coolant and in the region of the consumable part by comparison with the power supply unit and cooling unit. Consequently the region of the screw nipple represents a particularly endangered place.
Additional and substantial mechanical stresses result from the insertion of the electrodes, boiling distortion and also are due to scrap pieces which slide into the melt.
Due to the stringent operating requirements made on electrodes they require constant improvement. It is therefore the object of the invention to provide electrodes of high activity with a low current drop and low voltage in the supply lead, with the least tendency to be trouble prone but being also easy to manufacture and to repair. Particularly in cases of undesirable shift of the arc or excessive mechanical stresses such electrodes must allow the electrode arc process to be continued, even in the event of partial damage, in a manner which is improved compared with that of conventional arc electrodes.